1. Field of the Invention
The present invention relates to a finder optical system for use in a still camera, a video camera or the like, and more particularly to a Keplerian finder optical system which is compact in size and has a wide viewing field.
2. Related Background Art
Image viewing apparatus employing Keplerian optical system are already well known, as represented by telescopes. In such Keplerian telescope optical systems, there are employed various structures for obtaining an erect observed image, and there are principally known the following four structures as the erect image optical system:
(1) a system including an erecting prism on the optical path between the objective lens and the focal position thereof, as in prism binoculars;
(2) a system in which an inverted image on the focal plane of the objective lens is reformed as an erect image by a relay lens, as employed in ground telescopes;
(3) a system including a mirror between the objective lens and the focal plane thereof, and a pentagonal roof prism between said focal plane and the eyepiece, as in the finder system of single-lens reflex cameras; and
(4) a system including an erecting prism between the focal plane of the objective lens and the eyepiece.
However, the above-mentioned system (1) requires a considerably long focal length for the objective lens, so that it is difficult to obtain a wide viewing field with a relatively low image magnification, as required in the finder system. The system (2) is disadvantageous for obtaining a compact finder system, particularly a finder to be incorporated in a compact camera, because the entire length of the system is large due to the presence of the relay lens. The system (3) can provide a shortest length, but it involves difficulties in the manufacture, as the pentagonal roof prism, requiring extremely precise working, is indispensable. Also the system (4) is disadvantageous for obtaining a wide viewing field required for the finder, because the erecting prism positioned immediately in front of the eyepiece limits the apparent viewing field thereof.